For many years it has been known in the automotive arts to provide some form of temperature-responsive actuating device for the fan employed in the liquid cooling type of heat exchanging system. The temperature responsive control is provided to reduce the overall cost of operating the system by operating the fan only intermittently or at reduced speeds when the engine is operating at high speed, thus to reduce air flow at times when the operating temperature of the engine does not warrant the full cooling effect of the fan. By way of comparison, a direct drive fan uses 5% to 8% more fuel than the same engine equipped with a temperature responsive fan.
With the increasing number of power consuming accessories used in conjunction with automotive internal combustion engines it becomes even more necessary to conserve power and to drive the engine cooling fan only to the extent necessary at times when the coolant temperature, or some other aspect of engine performance requires it. It is to be realized that the cooling fan can require a considerable amount of power and it is accordingly advantageous to minimize fan speed when its full operation is no longer required, which also serves to minimize the objectable noise associated with high fan speed. Further, uncoupling the cooling fan at comparatively low engine temperatures serves to speed up the warmup of the engine.
Many commercially employed temperature-responsive engine cooling fans are not directly responsive to the temperature of the liquid coolant of the engine. Rather, they are often directly responsive to the temperature of the air that circulates through the radiator. In many instances such air temperature does not give a true indication of the coolant temperature. Although devices have been proposed that are directly responsive to the coolant temperature, these devices are often complicated and costly to produce.
A great number of temperature-responsive fan drives embody some form of element that has thermal properties for actuating the drive. Examples of such elements are bimetallic springs, metals having high coefficients of thermal expansion and wax pellets. Certain of these devices deteriorate with age or their thermal properties may change with age. In addition, in the case of engine cooling systems, a change in engine coolant may make it desirable to operate at high or lower temperatures. It is thus sometimes necessary to replace the temperature-responsive element in these instances.
Examples of prior art temperature-responsive fan devices are U.S. Pat. No. 3,262,528 to Weir (employing a silicone fluid-shear coupling and a bimetallic strip), U.S. Pat. No. Re. 24,157 to Johnson (employing an expandibile fluid and a bellows), U.S. Pat. No. 3,180,571 to Caroli (employing a bellows), and U.S. Pat. No. 3,228,382 to Stefan (employing a venturi throat and a coolant in the vapor phase acting against a fan clutch device.)
Although there is obviously a great deal of justification for the use of a clutch drive fan, a problem arises in a vehicle thus equipped when the vehicle owner wants to pull a travel trailer, utility trailer, boat or horse trailer or the like, or when light duty trucks are heavily loaded. This is because the slippage of the fan will be such that insufficient amount of cooling air is directed across the radiator and engine so as to keep it at the proper operating temperature. This situation is a prime cause of engine overheating and it becomes aggravated by hilly or mountinous roads. High ambient summer temperatures further complicate the situation, and may well force the vehicle owner to seriously consider the physical exchange of a temperature responsive fan for a fixed drive fan so that he can continue to use his vehicle to pull the trailer or carry the load.
It was as a result of this type of problem that the present invention was evolved, and by virtue of this invention, newly-manufactured vehicles equipped with a temperature-responsive clutch fan can now be equipped with a selectively operable clutch fan locking device such that optimum fan operation for a given set of operating conditions can easily be obtained.